Typical aircraft used in day-to-day airline operations are commonly supported by a plurality of compressible landing gear struts. These landing gear struts contain pressurized hydraulic fluid and nitrogen gas. Aircraft landing gear struts incorporate a shock absorbing technique of forcing hydraulic fluid through an internal orifice-hole within a compressible/telescopic strut. As the aircraft lands and the strut compresses, the internal volume within the strut is reduced. This reduction of strut volume causes the contained nitrogen gas pressure to increase. The hydraulic oil which is also contained within the strut is a non-compressible fluid. This pressure increase of the nitrogen gas is explained by Boyle's Law, named after the chemist and physicist Robert Boyle, who published the original law in 1662. Boyle's Law describes the inversely proportional relationship between the absolute pressure and volume of a gas, within a closed system.
There are many critical factors the pilot of an aircraft must consider, when determining if that aircraft is safe for landing. One of those is the “proper fluid servicing levels” within the landing gear strut. “Proper fluid servicing levels” can be described as the proper volumes of both the hydraulic oil and nitrogen gas, within the compressible, telescopic landing gear strut. Aircraft manufacturers determine, certify and publish the minimum volume limitations for proper strut servicing levels, for respective aircraft landing gear struts. Properly serviced landing gear insure that during each aircraft landing event, the aircraft landing gear shock strut's gas and fluid levels are sufficient to absorb the energy of the airplane's transition from descending flight, through the landing impact, and to a smooth roll-out along the airport runway.
Typically landing gear struts are serviced by the following procedure. While the aircraft is inside a hanger and completely lifted up off of the ground, by jacks; the landing gear are suspended below the aircraft wing or fuselage, thus support no weight. In this suspended position the telescopic aircraft landing gear struts will extend to their full telescopic limits. Lifting all of the aircraft weight off of the landing gear reduces the internal pressure within the landing gear strut to a minimal amount, commonly called the “pre-charge pressure.” That pre-charge pressure is then released through a nitrogen gas servicing valve, often called a Schrader valve. Once all of the internal pressure has been released, a secondary jack is placed beneath the landing gear strut in a position as to allow that jack to lift the landing gear strut thus collapsing the telescopic piston of the landing gear and compress the de-charged strut to its minimum extension position. Once in this position, hydraulic oil will be added to the strut (through the Schrader valve or some other fill-port) until such time as all volume within the totally collapsed strut if full of oil. The Schrader valve will then be closed and the jack will be removed from beneath the collapsed landing gear strut. At this point, a compressed nitrogen gas bottle will be attached to the closed Schrader valve and the Schrader valve will be re-opened. Compressed nitrogen gas will then be forced into the strut until such time as the internal strut pressure reaches the manufacturer's design “pre-charge pressure” (as an example and in this case the pre-charge pressure is 182.25 psi). The introduction of compressed nitrogen gas to the strut will force the collapsed strut to extend, and the telescopic strut will extend to its full telescopic limit. The aircraft will then be removed from the jacks which were supporting the entire aircraft. As the aircraft is removed from the jacks, the weight of the aircraft is transferred onto the landing gear struts. The landing gear strut will compress until it reaches a point of equilibrium where the internal strut pressure is sufficient to support the aircraft. With a tricycle design aircraft, the aircraft rest on three pockets of compressed nitrogen gas. Once the aircraft is resting on the hanger floor, the landing gear strut will have an extension amount where about four inches of the chrome telescopic piston is apparent. This is what most pilots call the “four inches of shiny” which they typically look for, and feel comfortable with, as they conclude the pre-flight walk-around aircraft inspection.
Airline Maintenance Operation Departments have the responsibility of insuring these landing gear struts are properly serviced, and are often divided into two separate divisions. These two divisions being: “Line Maintenance Operations” with the responsibility for minor maintenance procedures that may arise while the aircraft is located at the airport gate, just prior to departure; and the traditional “Maintenance Department” with the responsibility of more extensive maintenance activities, and regularly scheduled maintenance task and procedures accomplished within an aircraft hangar; during such times as the aircraft is removed from scheduled operations.
One issue for consideration, regarding this invention, often arises with the Line Operation Maintenance, while the aircraft is awaiting departure from the airport gate. As an example . . . Prior to departing the airport gate the co-pilot will perform an aircraft pre-flight inspection walk-around. During this walk-around he may notice one of the main landing gear struts has a posture where the strut appears to be collapsed/compressed more than would be typically acceptable (the pilot not seeing the 4″ of shiny chrome), especially when comparing it to the opposing main landing gear of that same aircraft. The co-pilot will report that strut as being low, to the Line Operations Maintenance Department and a service technician will be sent to investigate the strut. In most cases that Line Operations Maintenance technician will make an assessment, and conclude that the landing gear has lost nitrogen gas. The technician will then add additional nitrogen gas to that strut, until such time as the strut extends to a posture that appears correct. The typical “four inches of shiny” which pilots typically look for. The problem with this practice is that the nitrogen gas which is added to the strut is often replacing hydraulic oil which has leaked through the landing gear strut seals. The repetition of this procedure, on that same landing gear, over a period of several weeks can allow for a significant amount of non-compressible hydraulic fluid to be lost from that strut, and mistakenly replaced with compressible nitrogen gas. This will cause that strut to be less effective for landing. A landing gear with an insufficient amount of hydraulic fluid and an excess amount of nitrogen gas would have the initial compression of that landing gear, during the landing event, forcing nitrogen gas through the landing gear strut's internal orifice, instead of hydraulic oil; thus reducing the shock absorbing effectiveness of the landing gear strut.
This new invention describes two methods of determining if the proper volumes of both hydraulic oil and a third method of determining the lost volume of nitrogen gas from the landing gear strut. Where in the event described above, nitrogen gas was injected into a landing gear strut which had lost hydraulic oil . . . considering the aircraft is not suspended by jacks, the internal strut pressure is supporting the weight of the aircraft and the strut will maintain a high pressure. The only option for the maintenance technician is to inject enough volume of highly compressed nitrogen gas into the strut, until such time as the strut reveals the “four inches of shiny” which the pilot is looking for. This is the easiest and fastest way to insure the aircraft is not delayed from its scheduled departure.
Once the aircraft is airborne and no weight is being applied to the strut, the strut will extend to its full telescopic limit. With the strut now being over-serviced with an excess amount of compressible nitrogen gas, as opposed to a proper volume of non-compressible hydraulic fluid; the strut pre-charge pressure will be higher than what would be normally evident with a properly serviced strut.
Landing gear strut temperatures can vary widely from extreme low temperatures of a cold-soaked strut which has been flying at high altitude for 3-4 hours and kept at temperatures of −40° to times when the same landing gear is extended from within the aircraft wing and exposed to tropical climates and having winds as high as at 200 knots, passing across the landing gear generating friction and heating up the landing gear components. A pressure sensor which is temperature compensated can be used to help adjust strut pressure measurements for any errors caused by variable temperatures.
An alternative means of determining both nitrogen gas volume and hydraulic oil volume, is to use the aircraft landing event with the compression of the landing gear strut to force changes in internal strut volume, at the same time monitoring strut pressure changes, and compare both pressure and volume changes in relation to the passing of Elapsed Time.
Research of the prior art to determine aircraft landing gear strut servicing levels are documented and reference may be made to U.S. Pat. No. 4,092,947 Labrecque, U.S. Pat. No. 6,128,951 Nance and US Patent Application Publication US 2007/0069072 A1, Luce.
The prior art described by Labrecque explains of a mechanical apparatus added to the landing gear strut which incorporates a sliding rod which will rupture a disc at a selected position, to expose a low oil level indicator. This design differs from that of the present invention, as it is a mechanical device, utilizing no computer logic in its method.
The new art of this invention relates to improvements to the prior art of this inventor (Nance) U.S. Pat. No. 6,128,951. The prior art described by Nance, among other things, measures strut pressure within each landing gear strut, as well as the pressure distortions caused by strut seal friction. The prior art of Nance incorporate methods of using apparatus added to the aircraft which mechanically and physically changing the volume of fluid within the landing gear strut, while the aircraft is on the ground and at rest; while monitoring pressure changes in relation to pressure increases to a point where strut seal friction is overcome. As fluid volume within the strut is changed to induce strut movement, determining strut movement which has taken a longer period of time can be used to determine an excess in compressible gas within the strut.
The prior art described by Luce explains of a method of inserting flexible optical probes, through an external access port of the landing gear, to travel inside the mechanism of the telescopic landing gear strut, which visually identifies oil level within the strut. The art described by Luce application could be best utilized with aircraft landing gear designs of the future, due to the requirement to re-certify existing landing gear design to allow for the internal introduction of what would be a possible source of mechanism interference to the telescopic movement of the strut.
The art described within this current application is applicable as an uncomplicated re-fit to the existing landing gear designs, for the 44,000+ civilian aircraft and 27,000+ military aircraft currently in operation around the world today.
The new art described in this invention eliminates the need for apparatus to induce strut movement, or add complicated elements to be embedded within the landing gear mechanism, but instead collects pressure, temperature and movement data from external locations and servicing ports of the landing gear strut during an aircraft landing event. Strut pressure increases, compensated for changes in the strut's internal temperature, are monitored during the compression of the strut, in relation to Elapsed Time; and compared to previously stored data taken from a strut which was properly serviced. A properly serviced strut (which is now landing) will have the same “pressure-to-volume-to-Elapsed Time” profile as that of the pressure profile of a properly serviced strut which has been stored within the memory of the system. Any difference in the pressure profile will be detected and evaluated to determine the amount of gas-to-oil imbalance which has caused the pressure profile to be different from that of a properly serviced strut.